1. Field of the Invention
The gage diameter of the rock bit determines the size of the bit and the hole which it drills and is of primary importance in the drilling art.
The gage tolerance standard set by the American Petroleum Institute (API) is the nominal gage to plus 0.031 inch for bit sizes ranging from 33/8 to 133/4 inches and nominal gage to plus 0.062 inch for bit sizes from 14 to 171/2 inches in diameter.
During drilling operations on a drill rig, the drill string is often removed from the hole to change the drill bit type or replace a dull bit. Drill bit cutters, or cones, are designed to maintain the gage diameter even as wear occurs. If the hole diameter is undergage, the following replacement bit will have to ream its way to the hole bottom before starting to drill a new hole. Conversely, the gage of the replacement bit must be within the foregoing tolerance otherwise the new bit, if it is oversize, might not pass down the hole without jamming or causing damage to the well bore. Therefore, the rock bit must be manufactured to exacting standards.
Accordingly, this invention relates to the fabrication process of rock bits with a plurality of rolling cutters wherein the gage diameters must be held within close tolerances.
More specifically, this invention relates to an electronic means to monitor the external gage dimensions of rock bits, and also the internal gage diameter of core cutting bits during the assembly and welding process wherein the bit segments, comprising legs and cones, are joined, together or to a main body, by metallurgical bonding.
2. Description of Prior Art
Prior art methods to determine the gage of rock bits have traditionally utilized gaging rings that are slipped over the gage contact points of the rock bit cutters. The bit, comprising typically three segments which contain a cone mounted on the journal of each leg, is assembled into a clamping fixture for welding.
For example, U.S. Pat. No. 3,907,191 describes a ring gage which is positioned around the assembled bit segments, the individual segments are moved relative to one another causing the parting face of an individual segment to slide against the parting face of an adjacent segment. The segments are moved until the gage cutting surfaces of the cutters physically contact the ring gage, thereby insuring that the finished bit will have the desired gage size. These segments are then subsequently welded together over a substantial portion of the parting faces.
Such a method is disadvantaged in that the preset mechanical assembly is not monitored during the welding cycle hence the segments could move and any misalignment would not be discovered until the welded assembly is subsequently inspected. A rock bit which is out of gage must be dismantled and rewelded which, of course, is a costly and time consuming process.
The instant invention overcomes this difficulty in that the welding process may be continually monitored by, for example, an electronic eddy-current sensing system. The welding process can then be immediately stopped when an out of gage condition is detected. Corrections can be made before the weld has gone beyond a point in which the rock bit must be dismantled and rebuilt.
The accuracy and reliability of an eddy-current probe system are unaffected by nonconductive dirt films, electron beam energy or a vacuum condition within the welding environment. In addition, the circular confirmity or concentricity of the rock bit cutters may be continuously and individually monitored during the complete weld cycle.
An advantage is realized over the prior art in the use of a continuous non-contact measurement system wherein both the gage dimension and bit concentricity is electronically monitored during the welding process. Radial translation of the bit segments to gage and skew to proper journal offset is possible using all the foregoing systems. True bit geometry is assured by proper journal alignment in the bit segment carriers when the bit segments are initially assembled in the weld fixture.